This invention relates generally to reducing aircraft engine noise with a cancellation noise field which is generated by vibrational inputs to noise radiating structures and more particularly concerns changing the internal stress of the noise radiating structures to produce adaptive tuning of the resonant frequency of the structures.
Excessive noise has always been a problem confronting the aviation industry. Noise from discrete tones generated by the fans and turbines of modern aircraft engines is a major source of the noise problems. This is particularly true in communities surrounding airports which are subjected to the noise of take off and landing operations. Lately, fan and turbine noise has become an even larger issue due to the louder fan noise of the large-thrust high bypass engines and the increased enactment and enforcement of strict community anti-noise regulations.
Turbine and fan noise has been addressed for many years by employing passive techniques such as noise absorbing liners, tuned resonators, or a combination thereof. Such devices are usually mounted at the inlet and outlet of the engine to reduce noise radiated from the engine. Some disadvantages associated with these passive techniques include added weight and decreased thrust. Also, absorbent liners are generally ineffective against the long wavelengths of low frequency noise, and tuned resonators are only useful at the frequency to which they are tuned.
One possible active approach to fan and turbine noise control is to cancel the noise using secondary or cancelling noise fields generated with acoustical inputs. The cancelling noise field, which is of equal amplitude but 180.degree. out-of-phase with the primary field, destructively interferes with and cancels the primary field. However, standard acoustical inputs, such as loudspeakers, are expensive, fragile, heavy and require a relatively large amount of power. An alternative to acoustical inputs is disclosed in the copending application entitled "Active Control of Aircraft Engine Noise Using Vibrational Inputs," Ser. No. 08/051,810, filed Apr. 21, 1993 which is a File Wrapper Continuation of application Ser. No. 07/787,471, filed Nov. 4, 1991 and assigned to the same assignee as the present invention. Application Ser. No. 08/051,810 discloses using piezoceramic actuators instead of acoustical inputs to generate the cancelling noise field. The actuators are mounted either directly to an inner surface of an aircraft engine or to noise radiating elements resiliently mounted to the inner surface. When excited, the actuators produce vibrations in their supporting structure (the noise radiating elements or the engine itself) which "shake" the structure so that it generates the cancelling noise field.
The supporting structure which is shaken by the piezoceramic actuators may be made of low-damping materials because the peak vibratory response at resonance of elements fabricated with low-damping materials is very high and produces a correspondingly high acoustic output which is needed to cancel the engine noise. However, the off-resonance vibratory response of elements fabricated with low-damping materials is very low and does not result in an acoustic output sufficient to cancel engine noise. Thus, such piezoceramic-actuated noise cancellation is only effective over a narrow frequency range. Cancellation of aircraft engine noise is often required over a wider range of frequencies.
Accordingly, there is a need for an active noise cancellation system which is effective over a wide frequency range.